The present invention relates generally to circuit board structures, and more particularly, concerns a circuit board structure capable of accommodating sensitive analog signals and high speed digital signals in close proximity, without interference therebetween.
For illustrative purposes, the present invention will be described in terms of a circuit board containing a high density of high speed analog-to-digital converters in a single photon emission computed tomography (SPECT) scanner for nuclear medicine applications.
There are several distinct types of imaging systems in contemporary nuclear medicine. One type may employ gamma scintillation cameras (GSCs), so-called “position sensitive” continuous-area detectors, or simply, nuclear detectors. An exemplary GSC is a single photon emission computed tomography (SPECT) scanner. Another type of imaging system involves computed tomography (“CT”) of X-ray imaging. In contrast, magnetic resonance imaging (MRI) visualizes the inside of living organisms by making use of the relaxation properties of excited hydrogen nuclei in water placed in a powerful, uniform magnetic field.
Gamma rays are an energetic form of electromagnetic radiation produced by radioactive decay or other nuclear or subatomic processes such as electron-positron annihilation. Gamma rays form the highest-energy end of the electromagnetic spectrum. They are often defined to begin at an energy of 10 keV, a frequency of 2.42 EHz, or a wavelength of 124 pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard X-rays.
Gamma scintillation cameras, GSCs, are primarily used to measure gamma events produced by very low-level radioactive materials (called radionuclides or radio-pharmaceuticals) that have been ingested by, or injected into, a patient. The signals from the GSCs are used to generate images of the anatomy of organs, bones or tissues of the body and/or to determine whether an organ is functioning properly. The radiopharmaceuticals are specially formulated to collect temporarily in a certain part of the body to be studied, such as the patient's heart or brain. Once the radio-pharmaceuticals reach the intended organ, they emit gamma rays that are then detected and measured by the GSCs. Nuclear detectors perform spectroscopy and event X/Y positioning by processing signals from a constellation of Photo-multiplier Tubes (PMTs). An exemplary series of detectors comprises 59 PMTs.
To guard against the deleterious effects of stray X-rays blinding the scintillation crystal and the PMTs and possibly damaging the associated electronics, a GSC has a lead enclosure, i.e., tub, to block the stray X-rays. Furthermore, because the scintillation crystal emits faint amounts of light upon scintillation, the interior of the GSC must be shielded from ambient light that would blind the photosensors in the GSC used to measure the scintillation light emissions. The shielding guards against both gamma and X-rays.
Owing to the large number of interconnections between PMT preamplifiers and an acquisition electronics system, portions of the acquisition electronics system have typically been packaged inside the tub. It would be particularly desirable to perform analog-to-digital conversion, within the tub, of all the analog signals produced by the PMTs. This would permit a fully digital acquisition electronics implementation outside the tub, with all the attendant advantages. Space limitations have necessitated the use of a single board to perform the analog-to-digital conversion for all 59 PMTs. The board has sixty analog-to-digital converters, each with ten parallel digital outputs running at 30 MHz. With this dense packaging, such high digital speed signals tend to introduce digital noise to the sensitive analog circuits.